Methods of fabricating stents with enhanced fracture toughness

Abstract

Stents and methods of manufacturing a stents with enhanced fracture toughness are disclosed.

What is claimed is: 
     
       1. A method of making a stent comprising:
 radially expanding a polymer tube to about 400% to 800% of a starting diameter of the tube to increase the radial stiffness of the tube, wherein the fracture toughness of the radially expanded tube is enhanced by heating the tube to a temperature such that the crystal nucleation rate is at least twice the crystal growth rate while the tube is being radially expanded; and 
 forming a plurality of rings from the radially expanded tube, wherein a ring forms a plurality of cells, each of the cells being defined by opposed ends separated by a circumferential distance and opposed bending elements extending between and connecting the opposed ends, and wherein an adjacent pair of rings are interconnected by a linking element. 
 
     
     
       2. The method of  claim 1 , the polymer having a glass transition temperature (Tg) and a melting temperature (Tm), wherein the tube has a temperature of about Tg+0.2(Tm−Tg) when the tube is being radially expanded. 
     
     
       3. The method of  claim 1 ,
 wherein the tube is a poly (L-lactide) tube. 
 
     
     
       4. The method of  claim 1 , wherein the tube is a poly (glycolic acid) tube. 
     
     
       5. The method of  claim 1 , wherein the cell is a diamond shaped cell. 
     
     
       6. The method of  claim 1 , wherein a first cell and a second, adjacent cell share an end. 
     
     
       7. The method of  claim 1 , wherein the linking element is a longitudinal linking element. 
     
     
       8. The method of  claim 1 , wherein the linking element is connected to one of the ends that form the cell. 
     
     
       9. A method of making a stent comprising:
 radially expanding a polymer tube at a temperature in which the crystal nucleation rate is at least twice the crystal growth rate to both increase radial stiffness and enhance fracture toughness in the polymer tube; and 
 forming a plurality of rings from the radially expanded tube; 
 wherein a ring includes a plurality of cells, each of which being defined by opposed ends separated by a circumferential distance and opposed bending elements extending between and connecting the opposed ends. 
 
     
     
       10. The method of  claim 9 , wherein the rings are interconnected by longitudinal linking elements. 
     
     
       11. The method of  claim 10 , wherein an end forms an end for each of two, adjacent cells of the ring. 
     
     
       12. The method of  claim 11 , wherein a longitudinal distance separating the opposed bending elements define a maximum width for the ring. 
     
     
       13. A method of making a stent comprising:
 radially expanding an extruded poly (L-lactide) tube to increase its radial stiffness, wherein a temperature of the tube during the radial expansion is within a range where a crystal nucleation rate is higher than a crystal growth rate so as to provide at least a desired fracture toughness for the radially stiffened tube; and 
 forming the stent including forming a plurality of rings connected by linking elements from the radially stiffened tube; 
 wherein the stent is configured for being crimped to a balloon and then radially expanded within a body lumen by the balloon such that the inflated balloon induces plastic deformation of the stent to maintain the stent in an expanded state within the body lumen. 
 
     
     
       14. The method of  claim 13  wherein the temperature during radial expansion is about Tg+0.2(Tm−Tg). 
     
     
       15. The method of  claim 14  wherein the stent is laser cut from the expanded poly (L-lactide) tube.